When a large display or part of a display, color filter or another similar application, is produced, an exposure system transfer an image from a glass plate, preferably made from high quality quarts, onto a rather large substrate, which may have dimensions up to 1800 mm times 2400 mm. The exposure system includes an aligner, or stepper, that emits light through the glass plate and onto the substrate.
It is very important that registration of masks, i.e., the absolute placement in a coordinate system, is good enough to permit masks from different systems to fit together, e.g., the color filter and a TFT-array. Furthermore, large TFT-substrates may use one, two or more masks stitched together to cover a large exposure area.
A requirement for good alignment mark positioning determination in conventional registration system is inter alia a stable coordinate system, for instance a tradiational XY-coordinate system. A potential problem may arise when many positions have to be measured and the time to perform said measurement is significant, i.e., during a long measuring period drifts of any type may arise.